1. Field of Invention
This invention pertains to desalination of seawater and/or purification of polluted water by formation of methane hydrate at appropriate temperature and pressure conditions and subsequent formation of potable water therefrom.
2. Description of Prior Art
Original methods proposed for desalinating seawater involved distillation where seawater is heated to the boiling point and water vapor is then condensed to form fresh water. Distillation includes the use of sunlight to evaporate water and then collecting the condensate to form fresh or potable water.
Desalination by distillation was followed by the use of reverse osmosis which involves diffusion of fresh water from seawater through a semipermeable membrane due to the high pressure applied to the seawater feed tank. Desalination by reverse osmosis is considered more expensive than desalination by distillation primarily due to the cost of the semipermeable membranes and the high pressure pumps required.
Presently, desalination of seawater is effected by freezing. In indirect freezing, freezing is accomplished by circulating a cold refrigerant through a heat exchanger to remove heat from the seawater. Ice is formed on the heat exchanger surface and is removed, washed and melted to produce fresh water. In the category of freeze desalination by direct freezing, where desalination is carried out by the vacuum freezing vapor compression process, heat is removed from seawater by direct contact with a refrigerant. In a secondary refrigerant mode of this process, a refrigerant that has low solubility in water is compressed, cooled to a temperature close to the freezing temperature of salt water and mixed with seawater. As the refrigerant evaporates, heat is absorbed from the mixture and water freezes into ice.
Various alternative proposals for freezing desalination are described in paper entitled "Desalination by Freezing" by Herbert Wiegandt, School of Chemical Engineering, Cornell University, March 1990.
In gas hydrate or clathrate freeze desalination, a gas hydrate spontaneously is formed of an aggregation of water molecules around a hydrocarbon gas at temperatures higher than the freezing temperature of water. When gas hydrate is melted, fresh water and the hydrocarbon are recovered, thus simultaneously, producing fresh water and the hydrocarbon gas which can be recirculated. This approach has the advantage over other direct freezing processes in that the operating temperature is higher, thus reducing power requirements when forming and when melting the gas hydrates.
U.S. Pat. No. 5,553,456 to McCormack, discloses a clathrate freezing desalination system and method in which a clathrate forming agent is injected through a submerged pipeline to a predetermined ocean depth at which the surrounding ocean temperature is less than the clathrate forming temperature. The agent combines with the salt water to form a slurry of clathrate ice crystals and brine. The pipeline is concentric and coaxial with a surrounding outer pipeline in which the slurry is formed. The slurry is pumped back to the surface through the outer pipeline and the ice crystals are washed to remove brine. The washed crystals are then melted and the resultant water is seperated from the clathrate forming agent, which may be discarded or recycled for re-injection through the inner pipeline. The melting of the clathrate ice as well as the cold water and air circulating in the desalination plant can be utilized as a source of air conditioning for local buildings and facilities.
The clathrate forming agents disclosed by the U.S. Pat. No. 5,553,456 include carbon dioxide, halogenated methanes and ethanes, and cyclopropane. A clathrate is a generic term for an inclusion compound composed of water and other molecules of smaller size. Methane hydrate is a specific clathrate.